Many off-highway machines are driven by hydraulic motors mounted to planetary wheel drives. Some of these vehicles are capable of speeds of up to 35-40 miles/hour. Due to the large mass of these machines and their relatively high speed, braking is a very important function.
Braking is usually categorized in two ways: service (dynamic) braking and parking (static braking). On many high-speed off-highway machines, the service braking is handled by a combination of back-driving the hydraulic system and actuating a disc-caliper system on the output of the planetary wheel drive. Parking braking is usually done with a multi-disc wet brake at the input of the planetary wheel drive. This brake is usually spring-applied and released with hydraulic charge pressure.
There are shortcomings of a disc-caliper service braking system on high-speed off-highway hydraulic machines, namely:
due to the potential large momentum associated with these machines an external output disc-caliper service brake needs to be very large, which makes it a costly feature;
an output disc-caliper service brake is exposed to the environment, the environment for many of these machines can be severe and highly corrosive to a brake disc;
disc-caliper brakes also add considerable mass to a fairly weight sensitive application; and, for low to moderate braking, the hydrostatic system is used almost exclusively, this prevents the corrosion from being “wiped off” frequently by the caliper.
There are also shortcomings to wet, multi-disc input/intermediate service brakes, namely:
braking torque is transferred through gearing; and,
the brake becomes inherently long due to limitations in diameter and energy capacity needs;
To avoid making the assembly longer with input/intermediate brakes, the design would be compromised:
this would result in a recommendation of one-time use only before replacement;
may also result in limiting machine speed to reduce the potential energy that the brake may encounter; and,
the linear velocity of the disc pack is also increased (relative to the velocity of an output brake), this would be a large contributing factor in heat generated within an assembly and would be another reason to decrease machine speed.